Tech question: Using dummy load resistor with a speaker.

[Ed A]

Lately I have been gigging with a 50W Germino Lead 55 into a 1X12 cabinet with a vintage JBL D120… I love the tone but I’m concerned with throwing that much power at my fave D120 and possibly blowing it… So a couple of questions. I know that if I was running through two speakers, the power would be halved, 25W to each speaker instead of 50W to a single speaker… If I use a high-quality 100W 8 ohm dummy load resistor in the cabinet and connect it to my JBL, will that halve the power just as adding a real speaker would? I would put it in series giving me a 16 ohm total load. The next question is can I expect much change in amp performance or tone? In theory I thought this would be a great way of reducing power to the single D120, but don’t want to do it if there is some technical reason not to do it? Thanks!

 

[PaulD]

Simple answer: From a technical point of view there is no reason not to do it and it will reduce the amount of power being dissipated by the speaker but not necessarily by half (see below).

Slightly more complex answer: A speaker is a reactive load which means the actual impedance it presents to the amplifier at any given moment will vary over quite a big range and is frequency dependant, whereas a resistor is a purely resistive load and the load it presents to the amplifier will not change. This means that the power distribution between the speaker and the resistor will not always be 50/50.

As for how it would affect the sound there will likely be a slight reduction in perceived volume at any given amp setting and there may well be some effect on the tone. Tone is very subjective however and the only way to tell what the effect would be is to try it and see. Personally I doubt there would be much of a noticeable difference but far too many years of listening to loud rock music has done my hearing no favours!

You also need to be aware that the resistor will generate heat and could get quite hot (all that energy has to go somewhere). So you need to bear that in mind when considering where to mount the resistor.

 

[Ed A]

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[Ed A]

Simple answer: From a technical point of view there is no reason not to do it and it will reduce the amount of power being dissipated by the speaker but not necessarily by half (see below).

Slightly more complex answer: A speaker is a reactive load which means the actual impedance it presents to the amplifier at any given moment will vary over quite a big range and is frequency dependant, whereas a resistor is a purely resistive load and the load it presents to the amplifier will not change. This means that the power distribution between the speaker and the resistor will not always be 50/50.

As for how it would affect the sound there will likely be a slight reduction in perceived volume at any given amp setting and there may well be some effect on the tone. Tone is very subjective however and the only way to tell what the effect would be is to try it and see. Personally I doubt there would be much of a noticeable difference but far too many years of listening to loud rock music has done my hearing no favours!

You also need to be aware that the resistor will generate heat and could get quite hot (all that energy has to go somewhere). So you need to bear that in mind when considering where to mount the resistor.

Thanks for the detailed response! I certainly will listen and do an A/B comparison but happy to hear that it should give me the desired effect in terms of power dissipation… one other question, being that the resistor acts differently than the speaker, if I put this eight ohm resistor in series with the eight ohm JBL, I should be able to set the amp for 16 ohms, correct? I can take a reading with my ohm meter, but I know that’s not entirely accurate. Thanks!

 

[PaulD]

Yes that's correct, with the 8 ohm resistor in series with the 8 ohm speaker you would set the amp to 16 ohms.